A typical EDLC comprises two electrodes made of a nanoporous carbon material and interleaved with a porous dielectric film (separator). The entire porous system is impregnated with an electrolyte (in most cases, an aprotic, organic electrolyte) and hermetically sealed with current leads protruding out of the sealed case. EDLC devices based on most commonly used acetonitrile electrolytes can effectively operate at relatively low temperatures, with their upper temperature limit being 65-70° C. This is considered to be a drawback, because the upper temperature limit (e.g., for the applications mentioned above) should be increased up to about 100° C., so as to allow the EDLC to be disposed in proximity to, for example, an engine, or to be safely used in high-temperature environments. Moreover, the working voltage of 2.7-2.85 V, which is typical of most commercially available EDLC devices, should not be decreased at the increased temperatures so as not to lose the EDLC energy and power densities, which are both proportional to the square of the voltage.
In many cases, ionic liquids are considered as electrolytes for high-temperature/high-voltage EDLC devices; however, the high cost, limited availability and high viscosity of ionic liquids present drawbacks that prevent their practical use in EDLC technology.
Accordingly, there is a need to develop high-temperature electrolytes based on organic solvents, having low melting points, high boiling points and being capable of providing a high electrochemical stability and, correspondingly, a high working voltage for EDLC devices utilizing the selected electrolytes. However, an increase in the boiling point of a solvent often entails an increase in its viscosity and melting point, thus reducing the EDLC power capability and efficiency, as well as limiting its low operating temperature limit. For example, sulfolane, which has a fairly high boiling point of 285° C., has a melting point at 27.5° C. and a rather high viscosity of about 10 mPa·s near the melting point, thereby resulting in a low conductivity of electrolytes based on this solvent. On the other hand, most of the low-viscosity solvents have a boiling point far below 100° C., e.g., 81.6° C. for acetonitrile, the most popular solvent in EDLC technology due to its very low viscosity of about 0.35 mPa·s and, correspondingly, fairly high electrolyte conductivity at room temperature.
Another important issue that we have faced when selecting and optimizing the electrolytes for EDLC applications is the significant lowering of their electrochemical stability, and, consequently, a decrease of the EDLC working voltage with an increase in temperature. For example, sulfolane, mixtures of sulfolane with other sulfones and propylene carbonate and its mixtures with other carbonates tested in our lab demonstrated a decrease of the working voltage to 2.1-2.3 V if the temperature increased to 100° C. Unfortunately, the electrochemical stability of an electrolyte cannot be predicted a priori based on known properties of solvents and salts and so should be examined for each specific electrolyte.